1. Field of the Invention
The present invention relates to a digital subscriber line communicating system and a transceiver in the system which utilize an existing telephone line as a high speed data communication line. More particularly, it relates to an improvement of a modulation/demodulation system in a transmission apparatus used in the above-mentioned communication system.
In recent years, multimedia services such as internet and so forth have expanded through society and into homes. Accompanied by such development, it has been strongly required to promptly provide an economical and reliable digital subscriber line communicating system for utilizing such services.
2. Description of the Related Art
(1) An Explanation of an ADSL
As a technique to provide a subscriber line communicating system which utilizes the existing telephone line as a high speed data communication line, an xDSL (Digital Subscriber Line) is known. xDSL is a communicating system which utilizes a telephone line and a modulation/demodulation technique. xDSLs are generally classified into a symmetric type and an asymmetric type. In the symmetric type, upstream transmission speed from a subscriber home (hereinafter referred to as a remote terminal side or an RT side) to an accommodating central office (hereinafter referred to as a central office side or a CO side) is symmetric with the transmission speed from the central office side to the remote terminal side. In the asymmetric type, the transmission speed from the remote terminal side to the central office side is asymmetric with the transmission speed from the central office side to the remote terminal side.
In the asymmetric xDSLs, there is an Asymmetric DSL (ADSL) modem which is provided with the G.DMT standard having a downstream transmission speed of about 6 Mbit/sec. and the G.lite standard having a downstream transmission speed of about 1.5 bits/sec. Both of the G.DMT and G.lite employ Descrete Multitone (DMT) modulation.
(2) An Explanation of the DMT Modulation
DMT modulation will be explained using G.lite as an example. This explanation and the associated drawing will describe only the downstream modulation/demodulation from the central office to the remote terminal. However, DMT modulation is also possible in the upstream modulation/demodulation.
Firstly, transmitting data is input into an ADSL transceiver unit (ATU) in the central office and a one-symbol time (¼ kHz) of the data is stored in a serial to parallel buffer. The stored data are divided into a plurality of groups. A predetermined number of transmission bits per carrier signal is previously allocated to each group in accordance with a transmitting bitmap which will be described later in detail. Each group is output to an encoder. In the encoder, each group of the input bit series is converted into a signal point expressed by a complex number for an orthogonal amplitude modulation and is output to IFFT. The IFFT performs the conversion from each of the signal points to transmitting signal sequences by an inverse fast Fourier transform. The signals from the IFFT are output to a parallel to serial buffer. Here the sixteen points of the outputs of the IFFT are added as a Cyclic Prefix to the head of each DMT symbol. The output of the parallel to serial buffer is supplied to a D/A converter in which the digital signal with a sampling frequency of 1.104 MHz is converted into an analog signal. The analog signal is transmitted through a metalic line to a remote terminal.
At the remote terminal side, the analog signal is converted into a digital signal with the sampling frequency of 1.104 MHz by an A/D converter. Each DMT symbol of the digital signal is stored in a serial to parallel buffer. In the buffer, the Cyclic Prefix is removed from the digital signal, and the remaining signal is output to an FFT. In the FFT, a fast Fourier transform is effected to generate or demodulate the signal points. The demodulated signal points are decoded by a decoder in accordance with a receiving bitmap having the same values as those in the transmitting bitmap. The decoded data are stored in a parallel to serial buffer as receiving data of bit-sequences.
(3) A Detailed Explanation of the Bitmap
The bitmap described in the explanation of the DMT will be explained in detail with reference to FIGS. 13A and 13B.
The apparatus at the central office side and the apparatus at the remote terminal side both measure the ratio of the receiving signal to noise (hereinafter referred to as S/N) during a training period prior to communication to determine the number of bits to be transmitted by each modulating carrier. As shown in FIGS. 13A and 13B, for a carrier signal with a larger S/N, a larger number of bits to be transmitted are allocated; and for a carrier signal with a smaller S/N, a smaller number of bits to be transmitted are allocated.
By the above allocation, the receiving side measures the S/N to prepare the bitmap which indicates the numbers of bits to be transmitted corresponding to the carrier numbers.
The receiving side informs this bitmap to the transmitting side during a training period so that both the transmitting side and the receiving side can perform the modulation/demodulation with the use of the same bitmap during normal data communication.
(4) Countermeasure Against Cross-Talk from the Time Compression Modulation ISDN Transmission (Hereinafter Referred to as TCM ISDN Transmission)
When there is a cross-talk due to the TCM ISDN Transmission, in the prior art, two different bitmaps are used in the ADSL modem in the transmitting side or in the receiving side so as to improve the transmission characteristic. This method of using the two bitmaps will be explained with reference to FIG. 14.
In the TCM ISDN transmission, the central office side transmits downstream data during a prior half of one cycle of a reference clock signal of 400 Hz shown in (1) of FIG. 14, in synchronization with the reference clock signal of 400 Hz; and the remote terminal side receives the downstream data and then transmits upstream data. Therefore, the ADSL modem in the central office is influenced by a Near End Cross-Talk (hereinafter referred to as NEXT) from the downstream ISDN during the prior half of the one cycle of 400 Hz, and is influenced by a Far End Cross-Talk (hereinafter referred to as FEXT) from the upstream ISDN during the latter half of one cycle of 400 Hz.
Contrary to the central office, the ADSL modem in the remote terminal is influenced by a FEXT during a prior half of one cycle of the reference clock signal of 400 Hz, and is influenced by a NEXT during a latter half of the cycle.
If the metalic cable between the central office and the remote terminal is long, the S/N of the receiving signal to the NEXT is made smaller, and in some cases, the NEXT may be greater than the receiving signal.
In these cases, since the influence of the FEXT is not so large, in the prior art, two bitmaps are provided. One is a bitmap (DMT symbol X) for receiving signals during the NEXT period at the remote terminal. The other is a bitmap (DMT symbol Y) for receiving signals during the FEXT period at the remote terminal. During the NEXT period, in the prior art, the number of bits to be transmitted is made small so as to improve the resistance of the signals against the S/N. During the FEXT period, in the prior art, the number of bits to be transmitted is made large so as to increase the transmission capacity.
On the other hand, the time interval of one DMT symbol is usually 246 μs with a Cyclic Prefix of 16 points. Contrary to this, in the prior art, in order to synchronize the DMT symbols with the TCM Cross-talk period of 400 Hz, the time interval of one DMT symbol is made to be 250 μs with a Cyclic Prefix of 20 points so that one period of the TCM Cross-talk is made to coincide with the time period of ten DMT symbols, whereby the synchronization with the TCM Cross-talk is established.
(5) FEXT and NEXT
FIG. 2 is a timing chart showing the cross-talk that the ADSL receives from the TCM-ISDN.
The TCM-ISDN transmission is carried out at a frequency of 400 Hz with the period of 2.5 ms as shown in (1) of FIG. 2. During the first half cycle of each period of the TCM-ISDN, the CO side transmits symbols as shown in (2) of FIG. 2 and during the latter half cycle of the period, the RT side transmits symbols as shown in (3) of FIG. 2. In the first half cycle of the period of TCM-ISDN, therefore, the center ADSL unit (ATU-C) is affected by the near end cross-talk (NEXTC) from the TCM-ISDN, and in the latter half cycle, the ATU-C is affected by the far end cross-talk (FEXTC) from the TCM-ISDN as shown in (5) of FIG. 2. On the other hand, the subscriber ADSL unit (ATU-R) is affected by the FEXTR from the TCM-ISDN during the first half cycle of the one TCM-ISDN period, and by the NEXTR from the TCM-ISDN during the latter half cycle thereof. In this specification, the time areas affected by NEXT and FEXT in this way will be called the NEXT duration and the FEXT duration, respectively as shown in (4) and (5) of FIG. 2.
The center ADSL unit (ATU-C) in the CO side can estimate or define the FEXTR duration and the NEXTR duration at the subscriber ADSL unit (ATU-R) in the RT side. Also, the ADSL unit (ATU-R) in the RT can estimate or define the FEXTC duration and the NEXTC duration at the center ADSL unit (ATU-C) in the CO. That is, each period is estimated or defined as follows.                FEXTR: FEXT duration at ATU-R estimated by ATU-C        NEXTR: NEXT duration at ATU-R estimated by ATU-C        FEXTC: FEXT duration at ATU-C estimated by ATU-R        NEXTC: NEXT duration at ATU-C estimated by ATU-RThe transmission delay is also taken into consideration in these definitions.        
(6) Sliding Window
For the purpose of providing a digital subscriber line transmission system capable of transmitting the ADSL signal in satisfactory manner in the cross-talk environment from the TCM-ISDN described above, the present applicant has earlier proposed to introduce a “sliding window” in Japanese Patent Application No. 10-144913 (corresponding to U.S. patent Application Ser. No. 09/318,445 filed on May 25, 1999) which is incorporated herein by reference.
According to patent application Ser. No. 10-144913, in the downstream transmission of the ADSL signal from the center ADSL unit (ATU-C) to the subscriber ADSL unit (ATU-R), the state of the ADSL signal transmitted by the center ADSL unit (ATU-C) in the cross-talk environment from the TCM-ISDN is defined as follows.
That is, in the case where the transmission symbol is completely contained in the FEXTR duration, as shown in FIG. 3, the center ADSL unit (ATU-C) transmits the symbol as an inside symbol by means of the sliding window. Also, in the case where the transmission symbol is included in the NEXTR duration even partially, the center ADSL unit (ATU-C) transmits the symbol as an outside symbol.
According to the dual bitmap mode, the center ADSL unit (ATU-C) transmits the inside symbol using a bitmap A for the FEXTR duration and the outside symbol using a bitmap B for the NEXTR duration.
Similar to in the downstream transmission, according to the dual bitmap mode the subscriber ADSL unit (ATU-R) transmits the inside symbol using the bitmap A for the FEXTC duration and transmits the outside symbol using the bitmap B for the NEXTC duration in the upstream transmission.
Here, there is a case where the center ADSL unit (ATU-C) does not use the bitmap B. This case is a single bitmap mode. In such a case, the center ADSL unit (ATU-C) transmits only the pilot tone outside of the sliding window. In similar fashion, there is case where the subscriber ADSL unit (ATU-R) does not use the bitmap B. This case is also the single bitmap mode. In the single bitmap mode, the subscriber ADSL unit (ATU-R) transmits nothing outside of the sliding window.
As described above, an effective transmission technique under the noise environment from the TCM-ISDN has been proposed, for example, in Japanese Patent Application No. 10-144913 by the present applicant. Nevertheless, a specific training method for the ADSL transceiver in employing such a transmission technique or means for carrying out the training method have yet to be studied.